All of the FF replays are watchable with the lastest version of BW. However, it is possible that some of the replays aren't 100% "loyal" to the past reality due to be played on older versions (unit stats difference, used exploits that were fixed over the patches, etc.)

In addition, replays of games that were played on custom maps might have disync issues. It is recommanded to set the speed to "Fastest" and to keep it that way for the replay's duration for minimal disync.